In power converters having a phase-shift full bridge topology, the output filter inductor resonates with the output capacitance of the synchronous rectifiers on the secondary side when the current in the output filter inductor becomes discontinuous. The current in the output filter inductor becomes discontinuous when the current ripple exceeds the average output current. This condition is referred to as discontinuous conduction mode (DCM). Power converters typically enter DCM at light or very light loads.
The resonance between the output filter inductor and the output capacitance of the secondary-side synchronous rectifiers in DCM causes pre-charging of the output capacitance of the synchronous rectifiers to a certain voltage prior to the primary side of the converter starting a new power transfer interval. This stored charge in the output capacitance of the secondary-side synchronous rectifiers adds to the transformer reflected voltage on the secondary side, inducing a drain voltage overshoot at the secondary-side rectifiers. A higher voltage class of synchronous rectifier may handle the overshoot, but with worse figure of merit and therefore decreased overall system performance. Standard snubber techniques for mitigating the overshoot are lossy and over-dimensioned for any other working conditions of the power converter.
Hence, there is a need for an improved technique for mitigating the drain voltage overshoot that occurs on the secondary side of a phase-shift full bridge converter during DCM.